The extraction and use of finite energy resources and more recently the phenomenon of global warming have lead to interest in extracting energy from renewable energy resources such as wind, wave and tidal power.
The present invention is concerned with extracting energy from marine, ocean or tidal currents.
A number of different solutions exist for generating electricity through recovering the kinetic energy from ocean and tidal currents. The majority of these devices consist of submerged turbines to extract power from the moving mass of water. Some solutions involve mounting the turbine and generator nacelle off a pile placed into the seabed or other bottom fixed support structure for example International Publication Number WO 2004/048774. The disadvantage of the seabed or bottom mounted device is that the current velocity tends to reduce with depth below the water surface thus reducing the potential power capture. This type of bottom mounted device is also difficult to install as the support structure has to be fixed in place by a vessel or jack-up rig that can maintain its position in a sea area of strong tidal flow making marine operations difficult and dangerous. Also the requirement to use a jack-up vessel for installation means that deployment in deep water is complex, expensive and has high safety risks and this limits the number of tidal locations where this type of solution can be applied. Much of the world's tidal and ocean current energy exists in deeper water sites. Seabed fixed solutions require special means for efficiently extracting energy from bi-directional currents such as tidal currents and these can involve expensive and complex solutions such as variable pitch turbine blades or turbine and nacelle pods that rotate through 180 degrees with each change in tide cycle to efficiently extract energy from flow from the opposite direction.
Other devices involve supporting the turbine and generator with the aid of a surface floating body for example International Publication Number WO 88/04362, European Publication Number EP 1467091 and UK Publication Number GB 2256011. The disadvantage of a surface float is that it is subject to wave induced whole body motions which are transmitted to the turbines and generators and reduce the efficiency of the turbine and increase the design loads on the fixed and rotating machinery, particularly the turbine blades. A surface float also attracts large forces from the wave which increases the size and cost of the mooring system. In addition, the wave particle velocity is at a maximum near the sea surface and may act to counter the steady tidal or ocean current velocity thus reducing the efficiency of a turbine mounted near the surface as in WO 88/04362.
The object of this invention is to provide a moored floating body capable of supporting the turbine at a sufficiently deeply immersed draught such that the body will not experience severe motions in waves that could adversely affect the turbine performance and attract high mooring loads. The moored body should preferably use conventional mooring line components and thus minimise mooring and anchoring costs. Preferably, the device should weathervane on its mooring system such that the turbine always faces into the current and is not held on an adverse heading due to wave and wind forces that are not aligned to the current direction. This will enable a simple low cost fixed pitch turbine to be used with good efficiency as the flow into the turbine will always be from the desired direction. Alternatively, the device may be capable of being moored such that it is permanently aligned with the predominant directions of tidal current flow. In this case the turbine blades will be designed to accommodate bi-directional current flow, either by symmetry of section in the manner described in Japanese patent 07-293421 or by fitting a variable pitch turbine. Preferably the vessel should be capable of scaling up to a size that can support a large diameter marine turbine for maximum efficiency but preferably the vessel should also be easy to remove from its operating site where there are fast flowing currents to sheltered waters or onshore sites for maintenance. The installation and recovery hook-up operation should preferably be capable of being carried out over a short timescale, preferably over a period of slack water at either end of the tidal cycle, so as to minimise the cost and risk associated with these activities.
This invention relates to a partially submerged floating body that has special hull form and appendages leading to effective support for a submerged turbine for extracting energy from marine ocean or tidal currents. The configuration of the device offers low motion characteristics in waves making it particularly suitable for deployment in sea areas that experience waves in combination with marine currents. The invention also relates to features for controlling the heading of the device to avoid the build up of twist in the power export umbilical linking the moored and free to weathervane floating device to the seabed.
The body is used to house power generation, control, switchgear, transformer and other mechanical and electrical components necessary for the generation and export of electrical power. These components, while contributing to the functionality of the marine current power generation device, are industry standard solutions and not in themselves part of the present invention.
Accordingly, this invention provides a submerged buoyant structure that either directly houses, or acts as a support for additional pod structures that house the gearbox, generator and power conditioning electrical equipment. The deep immersion of the horizontal axis turbine or turbines attached to the submerged body or bodies ensures that turbine blades are in a clean current stream less disturbed by wave induced water particle motion and that they are less susceptible to cavitation due to the static pressure head of water above the blades. Attached to the submerged main body is a vertical strut or a plurality of vertical struts that pierce the water surface. The struts piercing the water surface are of small water-plane area such that they do not attract large changes in buoyancy with change in wave elevation.
Active lifting surfaces can optionally be appended to the submerged body or struts such that the lift forces generated by the appendages in a tidal current flow can be used to generate a stabilising moment to counteract any dynamic roll or pitch motion of the body. In addition the active lifting surfaces can, if correctly positioned at some distance from the mooring attachment point, be used to generate a trimming moment to counteract adverse trim induced by the vertical force component of the mooring system such that the device can be maintained at or near level trim to ensure optimum performance from the horizontal axis marine turbine.
The combined buoyancy of the submerged body (or bodies) with its vertical strut (or struts) together with any interconnecting structure is designed to be sufficient to support the total weight of the device in water plus the vertical component of load of any mooring and power export umbilical system while maintaining a certain freeboard of strut above the design waterline which provides reserve buoyancy. The device has sufficient reserve of buoyancy above the waterline to ensure the survivability of the device in waves, in so far as the body remains positively stable and does not permanently submerge. However, the device is designed to allow extreme waves to sweep over it in order to minimise mooring forces.
The surface piercing strut or struts are streamlined to ensure low drag for current flow parallel to the axis of the turbine and high drag for current flow normal to the axis of the turbine such that they cause the device to align itself on the heading of least drag of the submerged body and struts which will be with the current flowing into the turbine. For the device that is moored at a single point at the forward end this is by pivoting about its mooring system into the predominant current direction. For the device that is spread moored the moorings will be set up to constrain the device to align with the current flow.
In a further embodiment of the device additional struts or skegs can be fitted that extend down under the submerged body to improve its directional stability and to provide support for the body when placed on dry land or on the deck of a vessel for maintenance.
The semi-submerged geometry is such that the main body of the device that supports the turbine and the associated power generation equipment is suspended below the waterline such that the device attracts less wave induced loads than if it were surface floating. The small water-plane area surface piercing strut or struts attract relatively little wave loading. Multiple struts are configured to counteract any trimming moment induced by the mooring system and to provide pitch and toll damping through increasing the longitudinal and transverse stiffness.
The torque generated by the horizontal axis turbine is transmitted to the generator, either directly or through a step-up gearbox or some other means of interconnection such as a hydraulic drive. For the generator to deliver power the fixed windings or stator must be anchored to a body that will resist rotation. With this device the rotation of the submerged body housing the generator is resisted by the transverse hydrostatic stabilising moment created by the surface piercing strut or struts being rotated from the vertical. As an additional feature, as the reaction against the turbine torque is always in the same direction of rotation, and as that torque increases with the speed of flow of the marine current, it is possible to generate an opposing torque by constructing the vertical streamline strut with some asymmetry such that it generates a transverse lift force, which will also increase with current speed, and this lift force will generate a moment about the horizontal turbine shaft axis that will counteract the turbine generated torque.
An alternative turbine arrangement incorporates two counter rotating rotors of approximately equal diameter, each driving its own generator, such that the reactive torque is cancelled out. This would be a more complex and expensive solution than the mono-plane rotor but would have the advantage of having a higher overall turbine efficiency.
The embodiment of the invention that is moored at a single forward point incorporates a mooring system that allows the device to pivot about this forward mooring connection point such that it naturally aligns itself with the predominant current direction. By allowing the device to align itself with the current direction it is possible to use a turbine or propeller blade section that gives optimum performance in one design direction of inflow, this direction remaining substantially constant irrespective of the geo-referenced direction of current flow due to the self alignment characteristics of the submerged body, strut or struts and mooring system. The mooring system has a spread of mooring lines that are laid out in line with the predominant current directions to minimise the excursion of the device under the influence of the current and to offer best transferral of current induced drag into the mooring lines. Preferably, the mooring lines are pre-tensioned such that the downstream unloaded mooring line or lines always experience a degree of tension to avoid snatch loads in high sea states.
Allowing the device to pivot freely about its mooring system raises the complexity of how to avoid entanglement of the mooring lines with the power export umbilical. The preferred solution adopted with this invention is to feed the power export umbilical down through the centre of the mooring arrangement, a solution that is commonly adopted on single point moored Floating Production, Storage and Offloading (FPSO) platforms used in offshore oil and gas recovery. If the device were allowed to build up rotation in one particular direction it would eventually lead to twist and possible damage to the umbilical. This twist of the umbilical can be avoided by incorporating a power transmission swivel connector in the umbilical, such as a slip ring system, which again is a solution generally adopted for FPSO's. Such a system would require to be sealed against ingress of water. An alternative solution is to avoid build up of multiple rotations of the device by controlling the rotation of the device between changes in direction of the current. A novel solution which may be adopted with this invention is to use a lateral thruster that ensure that the device aligns itself during slack water such that when the current builds up the drag forces on the body rotate the device in a prescribed direction that avoids the device rotating through more than 360 degrees from its original setup alignment. An onboard control system may monitor the degree of rotation from the original alignment through the use of a pair of Global Positioning System (GPS) tracker mounted above the waterline and spaced apart from each other. With this solution there is no requirement for a power export umbilical or mooring swivel and the umbilical is gripped where it exits the device so that all of the limited rotation of the umbilical occurs outside the device where there is the greatest freedom of rotation of the umbilical.
In an alternative embodiment of the device, the device is spread moored on an orientation that aligns the device with the ebb and flood predominant tidal directions. This embodiment employs two or more point mooring lines where each mooring connection point is as dose as possible to the longitudinal extremities of the device. By attaching the moorings to the extremities of the struts this ensures that the mooring lines are kept clear of the turbine blades. In this embodiment the device can have a turbine fitted at either end of the submerged body thus allowing increased power capture to offset the reduced efficiency of a turbine blade section that is symmetrical about its mid chord in order that it can cope with flow from either direction.
Other elements of the mooring system include mooring connection shackles and seabed anchors. The seabed anchors can be of any type employed in the marine and offshore industries for restraining ships and floating offshore structures as appropriate to the seabed conditions. The first section of the mooring line at the device end will preferably be chain or weighted line such that the slack mooring line hangs free and avoids fouling the turbine blades.
The geometry of the submerged body plus surface piercing strut is arranged such that the still water centre of gravity aligns longitudinally with the centre of buoyancy of the device taking account of the additional trimming moment induced by the mooring system and that the centre of buoyancy is vertically above the centre of gravity thus ensuring the positive stability of the device. This is achieved through ballasting the device to the correct draught and trim by using both solid ballast and filled water ballast tanks. The water ballast tanks can be emptied using compressed air or ballast pumps so as to reduce the draught and alter the trim of the device in order that the mooring connection point becomes more accessible.